Group III nitride crystals (single crystals) such as gallium nitride (GaN) are widely used as the base substrates of electronic devices including high electron mobility transistors (HEMTs) and optical devices including light-emitting devices such as light-emitting diodes (LEDs) and light-receiving devices and as the functional layers exhibiting desired device characteristics in those devices.
A substrate formed of a group III nitride crystal (group III nitride substrate) is obtained by growing a group III nitride crystal on a base substrate made of a group III nitride crystal having the same or different compositions or a base substrate made of heterogeneous materials such as sapphire and silicon. The base substrate used in such a case may also be referred to as a seed crystal. Alternatively, in some cases, a group III nitride crystal is grown, and then, a base substrate is removed.
A well-known technology forms a GaN layer on a GaN epitaxial substrate obtained by forming a GaN layer on a sapphire substrate by a vapor phase method and then peels off the sapphire substrate, to thereby obtain a freestanding GaN substrate being a group III nitride substrate (for example, see Patent Document 1). Another well-known technology forms a group III nitride crystal on a seed crystal substrate by a flux method that is one type of liquid phase method (for example, see Patent Document 2).
In another well-known technology, before growing a group III nitride crystal on a GaN substrate by the metal organic chemical vapor deposition method (MOCVD method), to reduce an influence of polishing scratches existing on the surface of the GaN substrate on crystal growth, the GaN substrate is heat-treated at a temperature of 1100° C. or higher for 10 minutes or more under the atmosphere of a process gas containing ammonia and hydrogen in a MOCVD apparatus (for example, see Patent Document 3).
Another well-known technology forms an n+GaN layered region including silicon (Si) in an interfacial region when a GaN epitaxial film is grown on a freestanding GaN substrate (for example, see Patent Document 4).
In the case where a nitride layer is deposited to have a thickness of several to several tens μm on a GaN substrate by, for example, the MOCVD method so that a HEMT structure or an LED structure is laminated, a steep lamination interface having good crystal quality is needed to improve the device characteristics. For that purpose, the surface of the GaN substrate is required to be flat. Chemical mechanical polishing (CMP) is typically applied as the method for treating a GaN substrate surface, which is performed prior to layer lamination.
Unfortunately, an electronic device, formed by laminating a GaN layer on a GaN substrate after the CMP process by the MOCVD method, has not obtained characteristics expected from its design value. In particular, n-type carriers have been difficult to control.
To identify its cause, the inventors of the present invention have conducted analysis by secondary ion mass spectrometory (SIMS) to find that a high-concentration Si impurity layer exists at an interface between a GaN layer and a GaN substrate.
The following is conceivable as a contributing factor to the formation of such a Si impurity layer: abrasive grains (colloidal silica), which have adhered to the surface of a GaN substrate in CMPing the surface before the formation of a GaN layer and have not been completely removed after the subsequent cleaning process to remain as particles on the affected layer formed on the surface of the GaN substrate, and deposits, which have volatilized from a case or the like to adhere to the surface of the GaN substrate while a GaN substrate has been stored, diffuse when it is heated to have an elevated temperature in GaN layer formation.
The affected layer exists with a thickness of several to several tens nm from the surface of the GaN substrate, which conceivably contributes to Si diffusion. It is thus conceivable to perform the heat treatment as disclosed in, for example, Patent Document 3, to remove a high-concentration Si impurity layer when the affected layer is removed. In such a case, however, the flatness obtained through the CMP process becomes deteriorated, resulting in the degradation of device characteristics of LEDs or the like.